The present invention relates to pre-expanded particles of a polyethylene resin. More particularly, the invention relates to pre-expanded particles of a polyethylene resin which are applicable to a process for producing cellular molded articles by heating pre-expanded particles with a heating medium, such as steam, in a mold which can be closed but cannot be hermetically sealed.
Polyethylene resin foams are widely used as cushioning packaging material for high-grade goods such as office automation equipment, as a result of their good flexibility and their superior toughness and lack of brittleness compared to polystyrene and polypropylene resin foams. In these polyethylene resin foams, polyethylene resins such as crosslinked low density polyethylene, non-crosslinked linear low density polyethylene, and the like, can be used; however, since the resin densities of these kinds of low density polyethylene generally exceed 0.915 g/cm.sup.3, the obtained foams are too hard to use for softer cushioning applications, and in addition, have limited toughness. Improved flexibility can be obtained by using a copolymer, such as ethylene with vinyl acetate, which gives a softer pre-expanded particle. However, the melting point of this kind of ethylenevinyl acetate copolymer is low, with the result that the obtained foam has a poor heat resistance.
Japanese Unexamined Patent Publication (Tokkyo Kokai) No. 61-51008 discloses a polyethylene resin pre-expanded particle made using a crosslinked linear very low density polyethylene. However, when this kind of crosslinked linear very low density polyethylene is used, the melt viscosity of the obtained pre-expanded particles is increased and a higher pressure of heating medium is required during molding, compared to the case of using a non-crosslinked compound. This is disadvantageous with respect to the molding cost and cycle. Moreover, a further disadvantage is that recycling is difficult because of the crosslinking of the polyethylene.
The present invention has been realized in consideration of these kinds of problems inherent in the previous technology. Accordingly, an object of the present invention is to provide pre-expanded particles of a polyethylene resin, which can produce cellular molded articles having superior properties, such as flexibility, toughness, tensile strength, and so forth.